Attention has been focused on a technique for forming a transistor using a semiconductor thin film formed over a substrate (also referred to as a thin film transistor (TFT)). Such a transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another material, an oxide semiconductor has been attracting attention.
For example, a transistor including an oxide semiconductor containing indium (In), gallium (Ga), and zinc (Zn) as an active layer is disclosed (see Patent Document 1).
In addition, a technique for improving carrier mobility by forming a stack of oxide semiconductor films that is used for an active layer of a transistor has been disclosed (see Patent Document 2).
It has been pointed out that by entry of impurities such as hydrogen, an electrically shallow donor level is formed and electrons to be carriers are generated in an oxide semiconductor. As a result, the threshold voltage of a transistor including an oxide semiconductor is shifted in the negative direction and the transistor becomes normally-on, so that leakage current in a state where voltage is not applied to the gate (that is, in the off state) is increased. Thus, the entry of hydrogen into an oxide semiconductor film is suppressed by providing an aluminum oxide film having a property of blocking hydrogen over the entire region of a substrate so as to cover a channel region in the oxide semiconductor film, a source electrode, and a drain electrode, so that generation of leakage current is suppressed (see Patent Document 3).